Hot melt adhesives are widely used in the packaging industry for various applications. The most common applications are for case and carton sealing, and for tray forming. Such applications require resistance to fluctuations in temperature, especially for shipping packages from location to location. It is highly unacceptable for adhered packages to open because the adhesive either lacks cold temperature resistance and becomes brittle, or lacks high temperature resistance and softens. Brittleness will cause a shattering of the adhesive at the bond line and subsequent lack of fiber tear. As an adhesive softens with increased temperature, it may lack the green strength to sufficiently hold the flaps of a carton or case together, and the flaps "spring back."
In achieving high temperature resistance, cold temperature resistance is often sacrificed and the converse is also true. As high temperature resistance is increased, the products have a shorter open time resulting in less time for bonding, and also in decreased bondability due to the fact that there is less time for penetration into the substrate. Increasing high temperature resistance leads to more brittle products because the weight percent of the higher melting components, such as wax and tackifying resins with melting points of greater than 100.degree. C., are increased. The brittleness in turn causes decreased cold temperature resistance. It is therefore desirable to have products with high heat resistance that remain flexible enough for good cold temperature resistance, as well as a longer open time.
U.S. Pat. Nos. 4,816,306 issued Mar. 28, 1989 and 4,874,804 issued Oct. 17, 1989 to Brady et al. teach hot melt packaging adhesives based on ethylene n-butyl acrylate copolymers with terpene phenolic tackifying resins and synthetic high melting point wax. It is stated that this combination of components provide adhesives characterized by an excellent balance of high and low temperature performance without sacrificing machinability or thermal stability. There is no statement or suggestion that copolymers of ethylene and methyl acrylate may be used.
U.S. Pat. No. 5,331,033 issued Jul. 19, 1994 to Stauffer et al. also teach hot melt packaging adhesives based on ethylene n-butyl acrylate with aliphatic or cycloaliphatic or aromatic hydrocarbon resins or hydrogenated derivatives thereof, and a synthetic high melting point wax. It is also stated in this patent that the adhesives are characterized by an excellent balance of high and low temperature performance without sacrificing machinability or thermal stability. Again, these inventors did not contemplate the use of copolymers of ethylene and methyl acrylate.
U.S. Pat. No. 5,500,472 to Liedermooy et al. issued Mar. 19, 1996 teaches a hot melt adhesive based on an ethylene n-butyl acrylate copolymer with a terpene phenolic tackifying resin and a low melting point synthetic Fischer-Tropsch wax. It is suggested that small amounts, up to 20%, of other polymeric additives may be added including ethylene methyl acrylate copolymers in a list of others. This patent teaches adhesives with adjusted peel values of 130.RTM. (about 54.5.degree. C.) or less. The adjusted peel value indicates that some of the samples tested have fallen with the 10.degree. F. (5.5.degree. C.) increment as the temperature was being adjusted. These adhesives do not have superior heat resistance.
European Patent Application EP 0,721,006A1 published Jul. 10, 1996 teaches a hot melt packaging adhesive based on ethylene n-butyl acrylate copolymers having a melt index of at least 850 g/10 minutes combined with a rosin ester tackifying resin and a microcrystalline or paraffin wax, which may also contain ethylene vinyl acetate copolymers, ethylene methyl acrylate copolymers, ethylene acrylic acid copolymers, polyethylene, polypropylene, poly-(butene-1-co-ethylene), and lower melt index ethylene n-butyl acrylate copolymers as a non-essential ingredient. These adhesives tear fiber in the range of--35.degree. C. to 40.degree. C. The resultant adhesives are freezer grade adhesives having a slow rate of set and low peel values. The examples show adjusted peel values of 100.degree. F. (about 38.degree. C.). The adjusted peel values indicate that the sample failed in the 10.degree. F. (about 5.5.degree. C.) increments when the temperature was being adjusted. Even though this patent teaches ethylene methyl acrylate copolymers as one of many optional polymers that could be used, the patent also teaches that these formulations do not have high heat resistance, or formulations that have only cold resistance.
The present inventors have found a hot melt adhesive based on copolymers of ethylene and methyl (meth)acrylate which surprisingly have superior heat resistance over conventional packaging hot melt adhesives while maintaining a long open time and excellent flexibility and cold temperature resistance.